std::is_heap_until() algorithm

since C++20

// (1)
template< class RandomIt >
constexpr RandomIt is_heap_until( RandomIt first, RandomIt last );

// (2)
template< class RandomIt, class Compare >
constexpr RandomIt is_heap_until( RandomIt first, RandomIt last, Compare comp );

// (3)
template< class ExecutionPolicy, class RandomIt >
RandomIt is_heap_until( ExecutionPolicy&& policy, RandomIt first, RandomIt last );

// (4)
template< class ExecutionPolicy, class RandomIt, class Compare >
RandomIt is_heap_until( ExecutionPolicy&& policy, RandomIt first, RandomIt last, Compare comp );

since C++17

// (1)
template< class RandomIt >
RandomIt is_heap_until( RandomIt first, RandomIt last );

// (2)
template< class RandomIt, class Compare >
RandomIt is_heap_until( RandomIt first, RandomIt last, Compare comp );

// (3)
template< class ExecutionPolicy, class RandomIt >
RandomIt is_heap_until( ExecutionPolicy&& policy, RandomIt first, RandomIt last );

// (4)
template< class ExecutionPolicy, class RandomIt, class Compare >
RandomIt is_heap_until( ExecutionPolicy&& policy, RandomIt first, RandomIt last, Compare comp );

since C++11

// (1)
template< class RandomIt >
RandomIt is_heap_until( RandomIt first, RandomIt last );

// (2)
template< class RandomIt, class Compare >
RandomIt is_heap_until( RandomIt first, RandomIt last, Compare comp );

Examines the range [first; last) and finds the largest range beginning at first which is a max heap.

• (1) Elements are compared using operator<.

• (2) Elements are compared using the given binary comparison function comp.

• (3 - 4) Same as (1 - 2), but executed according to policy.

  Overload Resolution

  These overloads participate in overload resolution only if std::is_execution_policy_v<std::decay_t<ExecutionPolicy>>  (until C++20) std::is_execution_policy_v<std::remove_cvref_t<ExecutionPolicy>>  (since C++20) is true.

Parameters

first last	The range of elements to examine.
policy	The execution policy to use. See execution policy for details.
comp	Comparison function object (i.e. an object that satisfies the requirements of Compare). The signature of the comparison function should be equivalent to the following: bool cmp(const Type1 &a, const Type2 &b); The signature does not need to have const&, but must not modify arguments. Must accept all values of type (possibly const) Type and Type2, regardless of value category (so Type1& is not allowed, nor is Type1 unless for Type1 a move is equivalent to a copy (since C++11)) The types Type1 and Type2 must be such that an object of type RandomIt can be implicitly converted to both of them.

Type requirements

RandomIt

LegacyRandomAccessIterator

Return value

The upper bound of the largest range beginning at first which is a max heap.

That is, the last iterator it for which range [first; it) is a max heap.

Complexity

Linear in the distance between first and last.

Exceptions

The overloads with a template parameter named ExecutionPolicy report errors as follows:

• If execution of a function invoked as part of the algorithm throws an exception and ExecutionPolicy is one of the standard policies, std::terminate is called. For any other ExecutionPolicy, the behavior is implementation-defined.
• If the algorithm fails to allocate memory, std::bad_alloc is thrown.

Notes

A max heap is a range of elements [f; l) that has the following properties:

• Given N as l - f, for all 0 < i < N, f[(i - 1) / 2] does not compare less than f[i].
• A new element can be added using std::push_heap, in O(log(N)) time.
• The first element can be removed using std::pop_heap, in O(log(N)) time.

Examples

Main.cpp

#include <algorithm>
#include <iostream>
#include <vector>

int main()
{
    std::vector<int> v {3, 1, 4, 1, 5, 9};

    std::make_heap(v.begin(), v.end());

    // probably mess up the heap
    v.push_back(2);
    v.push_back(6);

    auto heap_end = std::is_heap_until(v.begin(), v.end());

    std::cout << "all of v: ";
    for (auto i : v) std::cout << i << ' ';
    std::cout << '\n';

    std::cout << "only heap: ";
    for (auto i = v.begin(); i != heap_end; ++i) std::cout << *i << ' ';
    std::cout << '\n';
}

Output

all of v:  9 5 4 1 1 3 2 6 
only heap: 9 5 4 1 1 3 2